Field of the Invention
The invention relates to a sintering plant having measures for decreasing dioxins in exhaust gas.
A sintering plant is a large-scale plant in the metallurgical industry and serves for agglomerating finely particulate or fine-dust metal, metal oxide or metal sulfide solid mixtures. The agglomerated solid mixtures can then be used in a blast furnace.
The material to be sintered, which is also referred to as sinter material, is solidified in the sintering plant by sintering, i.e. by heating to a point close to the melting point with surface softening and partial melt and slag formation. For that purpose, the sinter material is put onto a so-called traveling grate. In the case of ore smelting, the sinter material can be a fine ore, calcined pyrites, a blast furnace flue dust or a fine dust from a metal-processing industry, possibly together with returns, slag-forming addition or a solid fuel mixture, for example. The traveling grate in that plant is generally constructed as an endless chain from individual pallets to be filled. The chain runs over corresponding guide rolls in the working direction of the sintering plant. In that case, the filled upper track of the traveling grate or of the sintering belt runs over a suction box or forced-draft box, in which reduced pressure or overpressure is respectively generated by a suction or forced-draft fan, so that combustion air is respectively sucked or forced through the filled pallets. Sintering plants are currently known having grate widths up to 5 m and suction lengths up to 150 m.
At an inlet location, the pallets passing by are first charged with previously roasted returns to protect the grate bars and are then charged with the sinter material, which is possibly mixed with fuel. The charged pallets then pass by an ignition furnace, so that the sinter material or the fuel present therein is ignited by an ignition flame. The combustion and sintering process in the individual pallets then continues in a self-supporting manner because of self-sustaining combustion due to the air passed through by the suction or forced-draft fan, while the pallets are transported to an outlet location through the use of the sintering belt.
In order to conduct the exhaust gas or the combustion gas, a number of discharge lines are disposed along the upper track of the sintering belt in the sintering plant. The exhaust gas of the discharge lines transported by the suction or forced-draft fan through the individual pallets is collected in a collective exhaust gas line and finally passes through a stack into the environment. In order to recover fine dusts and to protect a downstream suction fan, an electrostatic precipitator is usually integrated into the exhaust gas line. The dusts produced from the electrostatic precipitator can be recirculated again to the sintering plant.
A majority of the sinter material processed in the sintering plant is finely particulate and fine-dust abraded material from filing, grinding or drilling processes of the metal-processing industry. However, that abraded material is frequently disadvantageously mixed with oily cutting aids, lubricants or coolants, which have halogenated hydrocarbons and aromatics to a not inconsiderable extent. Therefore, in the thermal processes during the sintering operation, the sintering plant also forms dioxins, which can pass into the environment through the exhaust gas. Sintering plants therefore represent the largest source for the emission of dioxins as compared with other large-scale plants of the metal production and metal-processing industry. Thus, in the exhaust gas of a sintering plant, dioxin values up to 60 ng TE/m.sup.3 have been found downstream of the electrostatic precipitator (TE=toxic equivalents). The term "dioxins" is used in this context as a collective term for the group of cyclic halogenated aromatic polyethers. They include, in particular, the cyclic ethers (furans) and the cyclic diethers (the actual dioxins). Particularly toxic representatives of the two groups which may be mentioned in this case are polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF).
In order to keep the level of dioxins released by a sintering plant into the environment below the legally prescribed limit value (currently 0.1 ng TE/m.sup.3), an article by W. Weiss, entitled: "Minderung der PCDD/PCDF-Emissionen an einer Eisenerzsinteranlage" [Reducing the PCDD/PCDF Emissions from an Iron Ore Sintering Plant], in VDI Berichte No. 1298 (1996), pages 249 ff., has disclosed adding a mixture of calcium hydroxide, Ca(OH).sub.2 and carbon (in the form of blast furnace coke or activated carbon) as an admixture to the exhaust gas of a sintering plant. That is done to adsorb the dioxins and to withdraw the partially dioxin-laden admixture from the exhaust gas again using a cloth filter and recirculating it back to the exhaust gas. In addition, an article by G. Mayer-Schwinning et al., entitled: "Minderungstechniken zur Abgasreinigung fur PCDD/F" [Techniques for Decreasing PCDD/F in Flue Gas Emission Control], in VDI Berichte No. 1298 (1996), pages 191 ff., discloses using zeolites as a dioxin adsorber in the exhaust gas of a sintering plant.
On one hand, in comparison with the adsorption technique, a substantially more effective catalytic breakdown of the dioxins in the exhaust gas of a sintering plant through the use of a suitable dioxin catalyst, due to the low exhaust gas temperatures of below 200.degree. C., is only possible with an expensive heating of the exhaust gas. Laden dioxin adsorbers, on the other hand, have to be ultimately disposed of in a landfill, which in turn gives rise to considerable costs and in addition represents a hazard to the environment.